Two different particulate materials can be separated from each other, if they can be charged differently and placed in an electric field. In one method of separation, corona charges are sprayed over a mixture of conducting and non-conducting particles flowing along the surface of a rotating metal drum. The charges sprayed on the conducting particles dissipate quickly through the drum (which is grounded) and are thrown off, while the non-conducting particles retain the charges and held to the drum surface by image forces. This method is referred to as electrodynamic separation, and is widely used for the beneficiation of potash and heavy minerals.
In another, a mixture of conducting and non-conducting particles is fed close to an electrode over a metal plate, which is grounded. The conducting particles are polarized in the electric field near the electrode and lose the charges of the same sign as that of the electrode to the metal plate, thereby acquiring a net charge opposite in sign to that of the electrode. The conducting particles are then lifted off the feed plate, while the non-conducting particles continue to move forward. This method is referred to as true electrostatic separation, and is widely used for separating strip wires from plastics and separating heavy minerals in beach sands.
In still another method, particles are contacted with a surface (e.g., the walls of a reactor) and acquire positive or negative charges depending on their work functions relative to that of the surface. By choosing a plate whose work function is in between those of the two different particulate materials to be separated, one can achieve separation. This method is referred to as triboelectrostatic separation. The U.S. Pat. No. 5,885,330 describes methods of using this technique for the removal of unburned carbons from fly ash. The same technique is also employed in the U.S. Pat. 5,755,333, in which triboelestrostatically charged particles are separated in a combined force field of electrostatic attraction and centrifugation. These methods were used for the beneficiation of fly ash.
The U.S. Pat. No. 3,407,930 and 4,274,947 showed that particulate materials also acquire triboelectric charges when they are agitated in a fluidized bed. The charged particles are then separated by creating an electrostatic field across the bed, the particles attracted by the electrodes being removed by means of a moving belt electrode. In the U.S. Pat. No. 4,839,032 and 4,874,507, particles acquire triboelectric charges by a vigorous agitation created by a belt moving in between two closely-spaced electrodes (less than 10 mm apart). The charged particles attracted to the positive and negative electrodes move toward opposite directions and are collected separately. Belt wears rather quickly; however, methods of minimizing it have been disclosed in the U.S. Pat. No. 5,819,946 and 5,904,253. These belt separators were tested for removing unburned carbons from fly ash.
In still another method, conducting particles are charged by contacting an electrode and are then separated from noncondcuting particles in an electric field. The U.S. Pat. No. 2,116,613 disclosed a method of feeding a mixture of particles of differing conductivities through a conducting chute electrified to a high potential, whereby a charge is acquired condcutively or by contact. The charged particles are then attracted by an oppositely charged electrode located underneath.
The U.S. Pat. No. 4,357,234 disclosed a similar method of charging particles and separating the charged particles from uncharged ones in an alternating current electric field of nonuniform intensity. In this invention, particles are fed to the surface of a flat electrode installed horizontally. An electromagnetic vibrator is installed underneath the horizontal electrode to move the particles forward. According to the inventors, the particles acquire charges either by triboelectrification or by conductive induction. The charged particles are then attracted toward the oppositely charged electrode. Since the electrodes are connected to an AC power supply, the charged particles oscillate between the two electrodes, i.e., they are in suspension. At the same time, the particles are subjected to a centrifugal force created by the nonuniform electric field, which in turn is created by installing the upper electrode with an angle to the bottom electrode. Thus, the charged particles move toward the direction transverse to the forward movement of nonconducting particles. Similar methods are disclosed in the U.S. Pat. No. 4,514,289 and 4,517,078. In the U.S. Pat. No. 4,556,481, the bottom electrode is made of sintered metal, so that air can be sparged to help suspend particles in the nonuniform electric filed.
The U.S. Pat. No. 5,513,755 disclosed a method of removing unburned carbons from fly ash by using a technique similar to that described in the foregoing paragraph, except that the electric field is created by a DC rather than an AC power supply. In this method, a fly ash feed is heated at a high temperature such that the surface temperature may be in the range of 250 to 600.degree. F. or higher. The heated fly ash is then fed to the upper surface of a conveyer belt, which is made of a conductive material, so that it can serve as an electrode. A counter electrode located above the belt electrode is shaped such that the distance between the upper and lower electrodes are larger the marginal edges of the belt than at the center of the belt. Such electrode geometry allows carbon particles move transversely of the belt movement, possibly due to the centrifugal force and the airflow caused by the ionization of the air in between the two electrodes. The electrical field in between the upper and lower electrode is higher than 2,000 V per inch. The lower belt is subjected to a low frequency mechanical vibration (100 to 800 impulses per minute), which is created using a multiplicity of rectangular beaters installed beneath the moving belt electrode. The mechanical vibration rearranges the orientation of the carbon particles so that they rise to the top of the layer of the particles by reason of their lightweight and, thus, become charged inductively. The charged particles are then subjected to the nonuniform electric field of separation.
The various electrostatic separation methods described above may be useful for removing unburned carbons from fly ash. They have inherent advantages over flotation in that the latter is a wet process, which entails high costs of dewatering. In 1996, the U.S. produced 59.6 million tons of fly ash, approximately 20% of which was recycled for productive use. Bulk of the recycled fly ash was used to replace pozzolans in Portland cement and as fillers in plastic and asphalt manufacture. The amounts of fly ash used in these applications are in the range of 15 to 35%. One of the problems in recycling fly ash as pozzolan is the amount of the unburned carbon left in it. Loss on ignition (LOI) is a common measure of the unburned carbon in fly ash, and the ASTM C114 describes a standard method of determining it. The unburned carbons in fly ash consume air-entraining agents used in concrete. They also affect pozzolanic reactivity and weaken the strength of concrete. Therefore, ASTM C-618-92a limits maximum LOI for Class F and C fly ashes to 6%. It is desirable, however, to further reduce the LOI of a fly ash preferably to below 3% using appropriate beneficiation methods to increase its marketability. The electrostatic separators described above are also useful for separating small amount of conducting materials mixed with noncondcuting materials, e.g., sulfide minerals present in siliceous tailings.